This application relates to Infrared Focal Plane Arrays, and more particularly, to a method for surface passivation that is resistant to aggressive processes. Focal plane array fabrication requires a well passivated material that is resistant to aggressive processes. Type-II InAs/GaSb superlattice heterodiodes have the ability to be more resilient than homo-junction diodes in improving sidewall resistivity through the use of various passivation techniques.
The growth of semiconductor III-V compounds by chemical vapor deposition (CVD) using organometallics and hydrides as elemental sources has recently developed into a viable process with many potential commercial applications. The metallo-organic chemical vapor deposition (MOCVD) process, based on the pyrolysis of alkyls of group-III elements in an atmosphere of the hydrides of group-V elements, is a common growth technique because it is well adapted to the growth of submicron layers and heterostructures.
Open-tube flow systems are used at atmospheric or reduced pressures in producing the III-V alloys. The process requires only one high-temperature zone for the in situ formation and growth of the semiconductor compound directly on a heated substrate.
Low pressure (LP-) MOCVD growth method offers an improved thickness uniformity and compositional homogeneity, reduction of autodoping, reduction of parasitic decomposition in the gas phase, and allows the growth of high-quality material over a large surface area. The LP-MOCVD technique has been successfully used to grow an InAsSb/InAsSbP alloy on an InAs substrate. InAsSbP alloys, which are potentially useful materials both for heterojunction microwave and optoelectronic device applications can be grown by liquid-phase epitaxy (LPE), molecular-beam epitaxy (MBE), conventional vapor-phase epitaxy (VPE), as well as MOCVD and MOMBE.
While each of the above processes are viable, certain disadvantages exist; for example, LPE experiences growth problems with InAsSbP alloys and potential nonuniform growth as well as melt-back effect. Molecular-beam epitaxy is a very expensive and complex process, and difficulties have been reported with p-type doping and with the growth of phosphorus-bearing alloys. Vapor-phase epitaxy disadvantages include potential for hillock and haze formation and interfacial decomposition during the preheat stage.
The technique of LP-MOCVD is well adapted to the growth of the entire composition range of InAsSbP layers of uniform thickness and composition on InAs substrates. This results first from the ability of the process to produce abrupt composition changes and second from the result that the composition and growth rate are generally temperature independent. It is a versatile technique, numerous starting compounds can be used, and growth is controlled by fully independent parameters.
The high quality of double heterostructure laser diodes based on the InAsSb/InAsSbP alloy on InAs substrate (100) grown by MOCVD is known which shows low threshold current density and a high output power. However, for the InAsSb/InAsSbP system it is important to have a system which can be reliably connected to other components of the device in which the laser is situated. The most common film used in passivation of III-V semiconductor devices is SiO2 for its simplicity in chemical properties. However, the properties of SiO2 film often leads to poor uniformity and film adhesion problems during processing.
Wet chemical etching methods are being used increasingly in the fabrication if III-V semiconductor lasers for mesa formation, pin holes and other pattern transfer steps. Most devices use dry etching techniques to obtain vertical sidewalls and deep mesas. Dry etching offers many advantages over wet etching for device fabrication, including better dimensional control and superior uniformity. The disadvantages are the much higher capitol equipment cost and, to a lesser extent, the question of damage in the semiconductor surface.
In semiconductor lasers, most applications often require lasers to operate in a single mode, generate a relatively high power condition or to provide special lasing characteristics. Lasers of this type are fabricated by specific design structure which often requires the etching of multiple semiconductor layers. The fabrication of index guided InAsSbP/InAsSb/InAs lasers is usually complicated and requires several stages.
The type-II InAs/GaSb superlattice has demonstrated the ability to perform infrared imaging in the Long and Middle Infra-Red Range (MWIR and LWIR). However, its performances are limited between 8 and 12 μm by its RoA and the absence of a passivation technique with the ability to efficiently protect the sample from degrading due to the aggressive steps in the Focal Plane Array (FPA) fabrication. The FPA process includes steps such as flip-chip bonding and underfill that apply considerable stress on the devices. Several passivation techniques have been proposed, but all of them present significant disadvantages. In the LWIR, SiO2 passivation layers degrade the electrical performances of homo-junction diodes by several orders of magnitude. Sulfide based passivation layers are too thin and volatile to mechanically protect the diodes. Epitaxial overgrowth of wide band gap AlxGa1-xAsySb1-y is complex and expensive.
Surface leakage is the consequence of the abrupt termination of the periodic structure of the superlattice that creates a bending of the bands at the surface of the semiconductor. In the proximity of the mesa sidewalls, this bending can provoke an accumulation of charges, eventually leading to a local inversion of the majority carriers in the superlattice. This may result in the creation of a strong current along the sidewalls, shorting the p-n junctions. The presence of process contaminants or native oxides can also modify the surface potential and create trap levels in the energy gap, increasing the trap-assisted tunneling currents. The passivation is supposed to reduce the surface leakage currents by fixing the Fermi level at the surface of the material, thus preventing any undesired bending of the conduction or the valence bands.
The surface passivation of type-II InAs/GaSb superlattice photodiodes has been a limiting factor to the incorporation of this technology in infrared imaging systems and has been a known process technology challenge for a number of years. The narrow band gap of long-wavelength infrared (LWIR) photodiodes and a highly reactive surface lead to Fermi level pinning at the surface of type-II sperlattice mesa diodes, resulting in deleterious surface-related tunneling currents and reduces, zero-biad detector impedance. Moreover, foreign absorbents and process contaminants can further alter the surface potential and introduce trap levels within the energy gap, leading to more efficient trap-assisted tunneling currents.
Demonstrating chemical, chalcogen-based passivation, dielectric passivation, and high band gap, semiconductor regrowth passivation has been demonstrated through surface passivation. The chalcogen-based passivation, though initially effective, does not address the practical issue of physical protection and encapsulation of the device, and there have been reports of the temporal instability of such a passivation layer. Dielectric passivation or type-II superlattice photodiodes presents the challenge of developing high-quality, low-fixed, and interfacial charge density dielectrics at process temperatures substantially below the material growth temperature, such that superlattice period intermixing does not occur. One solution is the subsequent regrowth of a lattice-matched, large band gap semiconductor over etched mesa diodes. This technique, however, requires very careful surface cleaning and preparation prior to the regrowth step and its feasibility in very high fill-factor focal plane arrays (FPAs) has not been demonstrated. This application presents an effective surface preparation and passivation technique, based on polyimide encapsulation, which aims to address the process technology challenges for type-II InAs/GaSb superlattice photodiodes and focal plane arrays.
Conformal polyimide (PI) physically passivates and protects the underlying semiconductor from the ambient environment while maintaining or even improving the electrical performance of the device measured prior to passivation. A number of groups have reported on the passivating behavior of polyimides in InGaAs/InP, GaAs, and AlGaAsSb/InGaAsSb/GaSb material systems and devices but no such investigations have been conducted with respect to InAs/GaSb superlattices nor on devices with band gaps smaller than ˜550 meV.